Wireless communication devices may be referred to as mobile telephones, user equipments (UE), wireless terminals, mobile terminals, mobile stations, cellular telephones, smart phones, laptops, tablets and phablets, i.e. a combination of a smartphone and a tablet with wireless capability. Wireless communication devices are enabled to communicate or operate wirelessly in a wireless communication system comprising multiple networks or Heterogeneous Networks (HetNet) with access nodes or access points. The heterogeneous networks may comprise, e.g. a cellular communications network comprising Second/Third Generation (2G/3G) network, such as Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access (WCDMA) or High Speed Packet Access (HSPA) etc., 3G Long Term Evolution (LTE) network, Worldwide interoperability for Microwave Access (WiMAX) network, Wireless Local Area Network (WLAN) or WiFi etc. for proving different type of radio access technologies (RATs). A wireless communications network may cover a geographical area which is divided into cells or cover areas, wherein each cell is served by a network node, which may also be referred to as a serving network node, an access node, an access point or a base station, e.g. eNodeB or NodeB.
The development of new generations of cellular systems simultaneously with upgrading existing generations allows for a wider range of accessible networks and RATs. Previously the preference of what system or network to use, given a choice, has most often been the latest one. At present that is often LTE. For operators it may be a benefit of being able to shift traffic from heavily loaded, possibly also less efficient, older networks to newer, less loaded and more capable new networks. However, as most of wireless communication devices are equipped with the most recent generation of system, steering all LTE capable wireless communication devices to an LTE network may not be the preferred method in order to optimize the total network performance.
In an environment where a wireless communication device has access to multiple networks with different RATs, the prior art method gives the wireless communication device an influence of a network selection by using a setting stating a preferred RAT, e.g., LTE (preferred)/WCDMA (HSPA)/GSM. Hence, as long as signals from the LTE network may be received, the wireless communication device will use that instead of one of the other networks, e.g., HSPA. In an environment where e.g., both LTE and HSPA co-exist, data rates for the two RATs are comparable. Furthermore, both LTE and HSPA allow for multi carrier signalling. In LTE this capability is denoted as Carrier Aggregation (CA), allowing for up to five LTE carriers to be aggregated, whereas in HSPA it is denoted as Multi Carrier (MC), allowing for up to eight HSPA carriers to be aggregated.
Another arising scenario today is multiple Subscriber Identity Modules (SIMs) devices which may carry two or more SIMs from a single or multiple operators in the same device. Particularly in Asia this has become de facto standard, although it has not been standardized by the 3rd Generation Partnership Project (3GPP). On many markets it is hard to get operator approval and volumes for a mid-end device without the capability of supporting at minimum Dual SIM Dual Standby (DSDS). The capability of supporting DSDS allows a UE to be camping on two cells simultaneously, or being connected to one cell and camping on the other. In case both SIMs are from the same operator, the UE may occasionally camp on the same cell but with two different identities and associated paging occasions. In order to qualify for high-end device approval, it is generally required to support Dual SIM Dual Activity (DSDA), whereby the UE can be independently connected towards two cells simultaneously.
The popularity of DSDS/DSDA devices on Asian markets depends on several factors. One factor may be that operators have different price plans e.g. for data and voice, or may have different price plans depending on calling subscribers in same or other network. Other factors may be, e.g. different coverage by different operators, i.e. spotty coverage, or that one cannot move a mobile phone number between operators. The trend is towards to support even more than two SIMs simultaneously, and devices with support for three and four SIMs, Triple SIM, Triple Standby (TSTS) and Quad SIM Quad Standby (QSQS) have been announced by some UE vendors.
For DSDA devices in active mode, it is required for the UE to use separate receivers for each connection, since it e.g. may use a Packet Switched (PS) service simultaneously for both SIMs, or may use PS service for one and a Circuit Switched (CS) service for the other. Therefore to support DSDS/DSDA, TSTS/QSQS and different RATs, the wireless communication devices usually comprise multiple receiving modules.
For improving the performance of wireless communication devices with multiple SIMs and multiple receiving modules, it is desirable to select a cell and RAT to camp on for respective SIMs when it is in idle operating mode and optimize the selection of cells and RATs for all SIMs.
In EP2613592, a method for single SIM UEs to obtain the best suitable RAT for use is disclosed. Parameters such as throughput and/or latency, bandwidth, Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ) may be measured for each of the available RATs, and the RAT with the best value in respect of high performance is selected.
However applying the same or similar principles for RAT selections for all SIMs in a wireless communication device with multiple SIMs may run into some problems. As in idle mode, a single receiver is usually used, i.e. radio resource or receiving module is shared between SIMs, for power saving reasons, independent selection of the best RAT for respective SIM may cause trouble in radio resource management of the wireless communication device during the idle mode procedures for respective connections.
In EP2605558, a method for RAT selection for a dual SIMs UE in idle mode for power optimization is disclosed. For example, for reducing power consumption, a first RAT may be employed that provides less bandwidth but uses less power to operate than a performance-centered second RAT. If both SIMs are idle, it may search for and discover RATs by indicia of the RATs such as RAT type (2G/3G/4G), signal strength, cell identifiers, or other indicia. As Radio Frequency (RF) interface or resource may be shared between the two SIMs when in idle mode, it may monitor for incoming calls for either SIM in accordance with a first RAT. During connected mode, the first RAT used in idle mode for the first SIM may be switched to the second RAT. One limitation of this method is that the first and second SIMs are forced to use the same RAT when both are in idle mode. The coverage provided by the selected RAT may be unfavorable for one of the first or second SIM when those are from different operators.
In WO2012055434, a method to operate a UE with multiple SIMs and Multiple Input/Multiple Output (MIMO) capability which comprises a plurality of radio branches in different operating mode is disclosed. In the first operating mode two or more radio branches are used to exchange information with a single network using single SIM. In the second operating mode the different radio branches are used to exchange information with different telecommunications networks using different SIMs. The first and second operating modes may be switched between depending on different requirements, such as data rate, reception quality or receiving and transmitting simultaneously in different networks. The method focus on connected mode and enable the user equipment to be flexibly used in dependence on the needs of the user and different reception situations. However, the cells and RATs to camp on for respective SIMs in idle mode may be unfavorable. Therefore the cell and RATs used for respective SIMs for the different operating modes may be non-optimal and once doing connection set up a handover may be needed which may increase signaling and risk of introducing interruption in connection, thereby quality of service may be lower.